Semiconductor micromechanical structures have a wide range of applications including sensors and actuators. Different structures can be used as accelerometers, flow sensors, pressure sensors, or micromovable tips, for example.
In many micromechanical devices, electrical elements are included in order to provide localized heating, piezoresistive stress sensing, capacitive position sensing, or electrical connections. Diodes or other active devices can also be included. These electrical elements can be formed by judicious doping of different types and levels or by metallization traces.
Many micromechanical devices have thin, high aspect ratio, structural components. High aspect ratio beams, for example, can be oriented and positioned to preferentially respond to forces acting in specified directions. In other words, a high aspect ratio beam will have a preferred direction of compliance.
For example, a vertically oriented high aspect ratio beam is compliant in a lateral direction, parallel with the plane of the substrate surface.
The broad, vertical sidewalls of a vertical beam (as shown in FIG. 1) cannot be patterned with conventional impurity doping techniques which are intended for doping horizontal surfaces. This prevents the fabrication of electrical elements on these sidewalls. For example, it is particularly advantageous to fabricate piezoresistors on the beam sidewalls to sense lateral deflections of the beam. Other electrical elements could be fabricated on the sidewalls to provide a wide range of functions, including electrical connections between sensors and electronic circuitry.
The prior art includes methods and apparatus for fabricating electrical elements on the sidewalls of stationary integrated electronic devices. Also, the prior art includes methods for using sidewalls as a means for controlling the distribution of ions implanted at an angle. For example, U.S. Pat. No. 5,200,353 describes a method for making a trench capacitor by doping the sidewalls of a trench with an angled ion beam. This method is limited to stationary integrated circuit structures and, more particularly, to trenches used for trench capacitors. Similar methods for applying angled ion implantation for making integrated electronic devices can be found in U.S. Pat. Nos. 5,512,498, 5,391,506, 5,021,355, 4,532,698, 5,444,013, 5,334,868, and 5,420,074. Generally, these inventions use angled ion implantation to improve the operation of integrated electronic devices such as trench capacitors, CCD arrays, transistors or the like. There is no mention of micromechanical devices in these references. They are not applicable to forming electrical elements on the sidewalls of micromechanical devices. Ion diffusion into sidewalls has been used in the prior art as an etch resist mechanism. It is well known in the art that doped semiconductor material can have different etch characteristics compared to the undoped material. Specifically, boron doped silicon is resistant to etching by certain silicon etchants. This phenomenon has been exploited by some researchers to prevent the etching of sidewalls of microstructures. For example, in "A novel etch-diffusion process for fabricating high aspect ratio Si microstructures", Technical Digest of the 1995 International Conference on Solid State Sensors and Actuators (Transducers '95), by W. H. Juan, and S. W. Pang, there is described a method for diffusing dopant atoms into the sidewalls of high aspect ratio silicon microstructures. This technique is concerned only with preventing the etching of the sidewalls so that the structure can be released. No technique is provided for controlling which regions of the sidewall receive dopant. Thus, this technique is unsuitable for fabricating electrical elements on the sidewalls. The fabrication of electrical elements on a sidewall requires control over where on the sidewall the dopant atoms are incorporated.
In the current state of the art, there is no method for fabricating electrical elements on the sidewalls of deformable micromechanical structures. Such a method would allow a greater range of micromechanical devices to be fabricated and hence for a greater range of applications for micromechanical structures.